The present invention relates generally to implantable devices for use within the cardiovascular system, and more particularly to deformable prostheses for implantation within and/or between blood vessels, and to methods of using them.
A variety of stents are known for use within arteries of a patient for treating stenoses, strictures, aneurysms, and the like. For example, a stent may be implanted within a partially occluded region of an artery to retain stenotic material beneath the stent and/or to open the lumen of the artery to improve blood flow therethrough.
Stents generally have a substantially cylindrical shape and are expandable between a contracted condition for facilitating delivery and an enlarged condition for engaging the vessel wall after deployment within the artery. Stents may be self-expanding, i.e., they may be biased to the enlarged condition but restrained in the contracted condition during delivery, for example within a sheath. Alternatively, stents may be substantially malleable or plastically deformable, i.e., the stent may be delivered in a contracted condition on a delivery catheter, and expanded by a balloon on the delivery catheter, until it plastically deforms into the enlarged condition.
Many stents include a plurality of segments or cells that are separated by one or more connectors extending between adjacent segments. For example, U.S. Pat. No. 5,104,404 discloses an expandable stent that includes a number of cylindrical segments, with single hinges connecting adjacent segments. Because of the rigidity of the individual segments of the stent, the hinges are intended to provide articulation between the adjacent segments.
When the stent is deployed within a curved portion of a vessel, the individual segments substantially resist bending to conform to the curvature of the vessel. The articulation provided by the hinges allows some conformity with the curvature of the vessel; however, the hinges may create gaps between the segments and/or may cause the segments to overlap one another. Material, such as stenotic material on the vessel wall, may extend through the gaps into the vessel lumen, possibly obstructing blood flow and/or breaking loose and traveling downstream where they may cause substantial damage to the patient being treated.
To reduce the likelihood of gaps occurring, some stents provide a number of connectors extending between adjacent segments. Increasing the number of connectors substantially, however, may increase the rigidity of the stent, which may cause problems during stent delivery. For example, when the stent is being delivered along a circuitous arterial path, the rigidity of the stent, particularly in its contracted condition, may impair advancement of the stent around tight bends in the artery.
Alternatively, some stents may include flexible connectors that are deformed when the segments are expanded to the enlarged condition. The resulting connectors may deform substantially to become part of the stent structure, i.e., they may deform substantially such that they lose their flexibility and are then unable to accommodate transverse bending forces.
In addition, some known stents have substantial gaps within the individual segments themselves or between the connectors, and so may not effectively xe2x80x9cscaffoldxe2x80x9d the underlying vessel wall, i.e., may not support the vessel wall to maintain a desired open lumen cross-section and/or may expose material extending from the vessel wall into the bloodstream. Initially, stents in their contracted condition may have substantially few gaps within the individual segments, i.e., peripherally about the circumference of the segments. When the stents are expanded to their enlarged condition, however, substantial gaps may be created at one or points along the circumference either within the segments, due to the design of the segments or to uneven expansion of the individual segments, or between the connectors.
The risk of uneven radial expansion may be particularly problematic with respect to balloon-expandable stents due to the nature of the balloons generally used. Balloon-expandable stents are typically manually compressed onto an inelastic balloon provided on the delivery catheter. Because of its inelasticity, the balloon is typically rolled circumferentially or otherwise wrapped around the catheter before the stent is placed over it, thereby attempting to ensure that the balloon is not snagged or damaged during delivery or deployment of the stent.
Once the stent is delivered intraluminally to a desired region within a vessel, the balloon is inflated to expand the stent to its enlarged condition. As the balloon unwraps during inflation, it may subject the stent to radial forces that are not substantially even along the length and/or the circumference of the stent. More particularly, some regions of the balloon may expand more quickly than other regions that have not yet fully unwrapped, causing localized heightened radial forces which may cause uneven radial expansion of the stent. Because of the unevenly distributed forces, a portion of the circumference of the stent overlying an initially unwrapped region of the balloon may be expanded greater than an adjacent portion where the balloon has not yet fully unwrapped. This may substantially increase the risk of over-expanding portions of the stent, and thereby creating gaps in the over-expanded portions.
In an effort to provide a uniform enlarged condition, stents generally have a substantially uniform pattern extending about the circumference of the individual segments, and generally have segments of equal lengths. Because of the uneven radial forces which may be encountered during expansion, however, these stents may not expand substantially uniformly despite the intended result of their uniform designs. This problem may be further exacerbated because individual stents are generally intended to be expanded to a range of potential enlarged sizes, for example, between 3.0 mm and 5.5 mm. While at the upper end of the range, the radial forces may become more even and expand the stent more uniformly, the stent may be prone to uneven expansion at the lower end of the range, where localized heightened radial forces are more likely to occur.
Accordingly, it is believed that there is a need for stents which more effectively scaffold the vessel wall and/or which substantially evenly engage vessel walls, particularly within curved vessel regions, and for methods and systems using such stents.
The present invention is directed to implantable devices for use within body passages, particularly within the cardiovascular system, and more particularly to deformable prostheses for implantation within and/or between blood vessels, and to methods of using them to create and/or maintain connections between adjacent blood vessels.
In one aspect of the present invention, a stent is provided for implantation within a body passage that includes a plurality of expandable segments defining a circumference and a longitudinal axis, and a connector extending between adjacent segments. Each segment includes an alternating pattern of curvilinear elements extending about the circumference.
In a preferred embodiment, the alternating pattern includes a first set of curvilinear elements having a first resistance to expansion and a second set of curvilinear elements having a second resistance to expansion substantially higher than the first resistance to expansion. Consequently, each segment is expandable between a contracted condition, a first or intermediate expanded condition, and a second or final expanded condition. Preferably, the first expanded condition is achieved when a radial force exceeding the first resistance to expansion is applied to the segment, and the second expanded condition is achieved when a radial force exceeding the second resistance to expansion is applied to the segment.
More preferably, the first and second sets of curvilinear elements are substantially xe2x80x9cUxe2x80x9d shaped elements having first and second longitudinal lengths, respectively, the second longitudinal length being substantially less than the first longitudinal length. The substantially xe2x80x9cUxe2x80x9d shaped elements of the first and second sets of curvilinear elements are connected to one another to define a substantially sinusoidal pattern extending circumferentially along the segments, the sinusoidal pattern having an alternating amplitude defined by the first and second longitudinal lengths. Alternatively, the substantially xe2x80x9cUxe2x80x9d shaped elements may have first and second thicknesses or diameters, wherein the first thickness is substantially less than the second thickness.
In addition, the connector preferably includes a curve extending at least partially circumferentially along the circumference defined by the plurality of segments. More preferably, the connector defines a sinusoidal shape adapted to extend and compress axially substantially evenly when the adjacent segments are subjected to a predetermined bending force. Although the connector may extend and compress, the connector preferably does not deform substantially when the stent is expanded, i.e., the connector remains substantially stationary and distinct from the adjacent segments and does not become part of the cellular structure itself. Furthermore, the connector preferably includes a pair of connectors located opposite one another on the circumference for facilitating articulation of the adjacent segments substantially transverse about the longitudinal axis.
Thus, an important aspect of the present invention is to provide a stent that includes a substantially tubular member plastically deformable between contracted and enlarged conditions, including a plurality of cylindrical segments and connectors for facilitating articulation of the adjacent cylindrical segments about the longitudinal axis, for example, when the stent is bent during delivery in a curved body passage and/or that substantially uniformly scaffolds the wall of the body passage.
In another aspect, the present invention is directed to a device for delivering an expandable stent, such as that previously described, to a site within a patient""s body. The device includes an elongate member having proximal and distal ends, a nose cone on the distal end, and an expandable member on the elongate member proximate to the nose cone for receiving an expandable stent thereon. The nose cone has a widened portion and a tapered distal tip to facilitate insertion along a body passage.
The stent delivery device also preferably includes an outer sheath slidable over the elongate member, the outer sheath including a lumen for receiving the elongate member therethrough. The outer sheath includes a distal end having a diameter substantially similar to the widened portion of the nose cone for substantially sealing the lumen when the distal end of the outer sheath engages the widened portion of the nose cone and/or to provide a substantially smooth transition therebetween to facilitate advancement of the stent delivery device through the body passage. In addition, the outer sheath may include one or more perfusion holes extending between an outer surface of the outer sheath and the lumen for allowing continued perfusion of fluid along the body passage during stent delivery. The nose cone may also include perfusion holes proximal and distal of the widened portion.
In addition, the stent delivery device also may include a shoulder on the elongate member proximate the expandable member. The shoulder preferably has a blunt distal edge for engaging a proximal end of an expandable stent received on the expandable member to prevent substantial proximal movement of the expandable stent. The shoulder may also include a substantially tapered proximal edge to facilitate withdrawal of the elongate member from a body passage.
The device may be used in a method for implanting a prosthesis or stent within a curved region of a body passage, the stent including a plurality of cylindrical segments and a plurality of connectors extending between adjacent segments, as previously described. The stent is placed in a contracted condition on a distal end of a stent delivery device, the distal end of the stent delivery device is advanced along the body passage, and the stent is positioned within the curved region. The stent is then expanded, first to an intermediate enlarged condition to substantially eliminate localized radial forces, and then further to a final enlarged condition, the circumferential pattern of alternating curvilinear elements expanding substantially evenly about a circumference of the stent to scaffold the curved region. The stent delivery catheter may then be withdrawn from the body passage, leaving the stent substantially permanently implanted within the curved region.
More preferably, the stent delivery device is used in a method for delivering an expandable stent to a selected delivery site within a patient""s body, for example, within the cardiovascular system. The stent delivery device includes an elongate member having an expandable member on its distal end, a tapered nose cone, a proximal shoulder, and an outer sheath for slidably receiving the elongate member therein. The stent is placed in a contracted condition on the expandable member, and the elongate member is inserted into the outer sheath to cover the stent. The distal end of the elongate member is advanced along a body passage within the patient""s body, and the stent is positioned at the selected delivery site. The outer sheath is withdrawn proximally to expose the stent at the selected delivery site, and the stent is expanded to an enlarged condition with the expandable member. The method described may be particularly useful for creating and/or maintaining a channel connecting a vein to an adjacent artery, preferably in the coronary system.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.